futex_unlock_pi() gets uval before taking the hb lock. Now imagine
someone in futex_lock_pi() took the lock. While futex_unlock_pi() waits
for the hb lock, the LOCK_PI sets FUTEX_WAITERS and drops the lock.
Now, futex_unlock_pi() figures out that there is waiter and invokes
wake_futex_pi() with the old uval which does not yet have FUTEX_WAITERS
set. This flaw lets cmpxchg_futex_value_locked() fail and return -EINVAL.
To address this race we could either use get_futex_value_locked() after
we obtained the lock but then we would need to handle the EFAULT case
(which I think means get_user() without the lock).
To avoid this I let wake_futex_pi() detect this and return -EAGAIN so it
can read the new value and try again.
As bad as this may sound: FUTEX_OWNER_DIED is not set while the owner is
unlocking the lock. FUTEX_WAITERS gets set by another task *but* this
means that the owner invoked the futex() syscall rather than doing a
cmpxchg() in userland (and libc does not do that).
Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
---
kernel/futex.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c
index 0d557e13823a..2911f54a6dde 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1496,8 +1496,12 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval,
struct futex_q *this,
if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
ret = -EFAULT;
- else if (curval != uval)
- ret = -EINVAL;
+ else if (curval != uval) {
+ if ((FUTEX_TID_MASK & curval) == uval)
+ ret = -EAGAIN;
+ else
+ ret = -EINVAL;
+ }
if (ret) {
raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
return ret;
@@ -2852,6 +2856,12 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned
int flags)
if (ret == -EFAULT)
goto pi_faulted;
/*
+ * Between get_user() and obtaining the hb->lock the uval
+ * gained a flag. Retry with the new value.
+ */
+ if (ret == -EAGAIN)
+ goto pi_faulted;
+ /*
* wake_futex_pi has detected invalid state. Tell user
* space.
*/
@@ -2883,6 +2893,8 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned
int flags)
spin_unlock(&hb->lock);
put_futex_key(&key);
+ if (ret == -EAGAIN)
+ goto retry;
ret = fault_in_user_writeable(uaddr);
if (!ret)
goto retry;
--
2.8.0.rc3
